The present invention relates to a process for applying a diffusion coating containing chromium and aluminum to a low carbon content steel article for improving its resistance to high temperature corrosion caused by combustion ashes rich in vanadium, sulfur and sodium. Low carbon steel articles with the aforementioned diffusion coating may be used in boilers and other petroleum burning devices.
There exists in the prior art several steels and alloys recommended for use in applications involving corrosive environments at high temperature conditions. There also exists several suggested metallic coatings for the protection of iron base materials. These suggested alloys and/or coatings are generally effective in applications involving oxygen and/or sulfur containing atmospheres and/or ashes containing a low concentration of vanadium. Most of these alloys and coatings are ineffective in situations involving highly corrosive ashes at high temperatures produced by the combustion of heavy and extra-heavy oil residuals having a high content of vanadium, sulfur and sodium in the ranges of from 400-1200 ppm, 3-6% by weight and 0.05-0.6 lb/bbl, respectively. In part this is because some of the coatings exhibit a microstructure with large grains oriented perpendicularly to the matrix coating interface. This type of crystallization, termed basaltic, compromises the good mechanical properties of the coated steel.
Some of these steels, alloys and coatings recommended for high temperature corrosion service were placed in contact with a salt mixture consisting of 85 wt.% V.sub.2 O.sub.5 and 15 wt.% Na.sub.2 SO.sub.4 and evaluated in a gaseous environment consisting of 2400 ppm SO.sub.2, 100 ppm SO.sub.3, 1% O.sub.2, and the balance nitrogen at a temperature of 700.degree. C. for 100 hours. These materials generally demonstrated a low chemical resistance. FIG. 1 shows typical weight loss results due to corrosion for these materials.
There are many techniques for forming diffusion coatings on metals. One such technique is known as pack cementation. Most pack cementation techniques have one thing in common--direct contact between the reactive mixture and the articles to be treated. U.S. Pat. Nos. 2,816,048 to Galmiche, 3,900,613 and 4,096,296 both to Galmiche et al., 4,141,760 to Baldi, 4,276,088 to Zaets and 4,293,388 to Rose et al. illustrates some of the pack cementation techniques known in the art for forming diffusion coatings.
The Galmiche '048 patent relates to a process for forming a chromium alloy on a body having as a principal component a metal selected from the group consisting of iron, nickel and cobalt. The process broadly comprises heating a mixture containing chromium and an inorganic fluorine compound to form chromium fluoride vapors and bringing the vapors in the presence of hydrogen into contact with the metal body at a temperature above 600.degree. C. while maintaining the metal body out of contact with the cementatious mixture. In an alternative embodiment, the mixture also contains a material selected from the group consisting of alumina, aluminum, zirconia, zirconium and silicon.
The Galmiche et al. '613 patent relates to a method for forming a surface diffusion alloy by applying at least one application metal selected from the group consisting of chromium, titanium, tantalum, vanadium, boron, aluminum, zirconium, and alloys thereof to the inner wall of a hollow metallic part. The method comprises introducing a cement comprising a powder of the at least one application metal, an inert diluent, a halogen or halogenated compound, and a surface active agent in an amount sufficient to confer thixotropic properties on the cement in the cavity of the hollow part, subjecting the cement within the cavity to vibration to temporarily liquefy it, heat the cement to generate vapors of the application metal to form a surface diffusion alloy of the application metal and the metallic part at the inner wall, and subsequently removing the cement using an aqueous or alcohol acid solution.
The Galmiche et al. '296 patent relates to a process for forming a surface diffusion alloy using one or two addition metals on a metal article. The process comprises at least partly covering the article with a metallic felt coated with a film of stable refractory oxide such as chromium oxide, submerging the article and the felt in a reagent containing a mixture of a first powder of the addition metal or alloy and a second powder of an inert diluent, and thereafter subjecting the reagent and the article to a thermal treatment in a halogen-hydrogen atmosphere at a temperature between 750.degree. and 1200.degree. C. The process is intended to improve the performance of hollow turbine blades which are subjected to hot corrosive gases.
The Baldi patent relates to the coating of metals such as plain carbon steel to improve their corrosion resistance. The coating may be formed by immersing the articles to be treated in an aluminum-alumina powder pack, heating the immersed articles to form an aluminized surface, spray an aqueous dispersion containing leafing aluminum on the article, baking the sprayed articles, forming a spray coating of colloidal alumina, and finally applying a top spray coating.
The Zaets et al. patent relates to a composition for forming a diffusion coating on ferrous metals. The composition contains 70 to 82% titanium, 14.5 to 20% alumina, 2 to 5% ammonium halide and 1 to 2% graphite. The coating is intended to protect work pieces subjected to highly concentrated salt solutions and chloride containing media such as pump and valve casings.
The Rose et al. patent relates to powder-type diffusion coating compositions for use in a codeposition process involving the formation of a diffusion coating of chromium and aluminum on high nickel and high cobalt superalloys. The compositions are characterized by use of the intermetallic compound Co.sub.2 Al.sub.9. The composition is particularly useful on superalloys containing about 16% chromium or less.
It is believed that these particular diffusion coating techniques are not particularly well suited to solving corrosion problems arising from burning petroleum fuels, especially those containing a high concentration of vanadium, sulfur and sodium. Pipes and pipe supports for boilers and other petroleum burning devices are generally formed from steel and often encounter aggressive environments containing such low melting salts such as V.sub.2 O.sub.5, Na.sub.2 SO.sub.4, vanadium complexes of sodium and sulfur ferrous alkalis under high temperature conditions. Consequently, these articles need to be formed from a material such as steel capable of withstanding these aggressive environments.
Accordingly, it is an object of the present invention to provide a process for improving the high temperature corrosion resistance of a ferrous article.
It is a further object of the present invention to provide a process as above capable of forming a high temperature corrosion resistant chromium-aluminum diffusion coating on a ferrous article.
It is yet a further object of the present invention to provide a ferrous article suitable for use in devices for burning heavy and extra heavy petroleum fuels.
These and other objects and advantages will become apparent from the following description and drawings